STRUCTURE OF THE SILICATES1.Approx. 90% of the mineral content of the earth’s crust is of silicates where Si-O bonding, coupled with different cations and anions, formed different minerals.2.The fundamental unit on which the structures of all silicates are based consists of four O2- apices of a regular tetrahedron surrounding and coordinated by one Si4+ at its centre.
STRUCTURE OF THE SILICATES3. The Si-O bond is partly ionic (generated due to attraction of opp. charged ions) and partly covalent. This tetrahedral groups can be linked with adjacent tetrahedral groupings through sharing of one or all four oxygen atoms (Polymerization).4. The mean Si-O bond length is 1.62Å. Other cations near the oxygen atom of a Si-O bond also attract the oxygen and tend to lengthen the Si-0 bond( greater co-ord no; longer bond length). The strength of the Si-O bond limits the range of the bond lengths from 1.60Å to 1.64Å
STRUCTURE OF THE SILICATESThe bond length between Si atoms and the bridgingThe bond length between Si atoms and the bridgingoxygen atoms (OBR) are on average longer by about0.025Å compared with the Si-O bond length to the O nb.[especially when SiO4 are linked in structures.The OBR-Si-OBR bond angle is also shows slightlysmaller value when bridging oxygens are not involved.This suggests that that the Si atoms are displacedfrom the centres of the tetrahedra (awayfrom O BR) cozof repulsive force between two Si atoms.The aforesaid sharing of oxygens gave rise to diversestructural configurations for silicates with various Si-Oratios.
When tetrahedra are corner-linked, the Si-O bond angledefines the orientation of the tetrahedra relative to oneanother. This bond angle can vary between 1200 and 1800depending on the local structural environment as well astemp. and pressure. The bond angle of a strain-free Si-O-Si bond is near 1400.When Al substitutes Si in a tetrahedron, the [AlO4]tetrahedra is slightly larger than a [SiO4] tetrahedra coz.Al-O bond (1.75Å) is larger than the Si-O bond. WhenSiO4and AlO4 are linked in a structure, this sizedifference is accommodated by a change in the T-O-Tbond angle (T= tetrahedral cation)
SiO44- complex ion• A group of ions that is so tightly bound together that they act like a single unit.• Building block of silicate minerals• 1 silicon ion + 4 oxygen ions arranged in a triangular pyramid • Electrical charge of -4
Tetrahedron Viewing Top point baseView from the top, Flat base of tetrahedron Side viewlooking down. facing you. Top pointTop point of of tetrahedron pointingtetrahedron away from you.facing you
Nesosilicates (island silicates)SiO44- tetrahedron forms ionic bonds with cations such as Mg2+, Fe2+ Example: Olivine Mg2SiO4 - Fosterite Fe2SiO4 - Fayalite (Mg, Fe)SiO4 Solid solution Mg2+ or Fe2+
Olivine: nesosilicate structure (island silicate) View from the side (“wall” of structure)Why Mg OR Fe?• Same size• Same electricalchargePeridot
Inosilicicates: Single Chains (Pyroxene)SiO3 Chain forms ionic bonds with cations above the tipand below the base Boxed region when view from How many Si How many O 2 Si 6O Yellow tetrahedron in front Gray behind and to the side Cation, forming ionic Stand here bond with tetrahedron look up the chain chain
Corner Sharing Tip to Tip O2- Si4+tesy of Donna Whitney, University of Minnesota Dept. Geology Face sharing
Nesosilicates: independent SiO4 tetrahedra b c projection Olivine (100) view blue = M1 yellow = M2
Inosilicates: single chains- pyroxenes b a sinβDiopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
Inosilicates: double chains- amphiboles Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 M1-M3 are small sites M4 is larger (Ca) A-site is really big Variety of sites → great chemical range Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na) little turquoise ball = H